1. Field of the Invention
The invention relates to X-ray inspection systems for examination of items such as baggage or packages. More specifically, the invention relates to an X-ray inspection system and method, that utilizes X-ray radiation modified by the item under inspection to detect, for example, weapons, drugs, explosives, or other contraband.
2. Discussion of Related Art
X-ray baggage inspection systems typically operate by exposing an item of baggage to X-ray radiation and detecting the X-ray radiation that is transmitted through or scattered from the examined baggage. Some systems have used a single view source detector arrangement, while others have used dual view or multi-view arrangements. The single or dual view systems usually scan baggage as it moves on a conveyor, using a fan or scanning pencil beam of X-ray radiation in a fixed geometry. Multi-view systems such as Computed Tomography (CT) systems usually perform a 360° scan of stationary baggage, and process data corresponding to absorption of the X-ray radiation from different scan angles to reconstruct a three-dimensional image of the baggage.
At airports, the baggage inspection procedure may be divided into a number of levels of inspection. A level one system may process baggage rapidly, such as at a targeted rate of approximately 1500 bags per hour. The level one system may be located at a first inspection station and may inspect all baggage. The level one system may rapidly scan baggage using some detection methodology, to eliminate non-suspicious baggage. This methodology may determine a property of materials within the baggage, such as, for example, mass density, or effective atomic number, or may employ Compton X-ray scatter, ion mass spectroscopy, or other detection techniques. The number of bags that are not cleared (that are rejected) by a level one system may range from 10%–50% of the total number of bags, depending on the detection methodology and threat thresholds used in the particular system.
In a multi-level system, the bags rejected by the level one system may be automatically sent to a level two area where an operator may visually inspect an X-ray image of the bag. The operator may search the image of the bag for characteristic objects, such as weapons, wires, explosives, etc., and may attempt to determine whether a suspicious object within the bag may be cleared based on its obvious shape. The operator at a level two station may clear most, but not all of the rejected bags. The remaining baggage may be on the order of, for example, 0.1%–0.5% of the initial stream, and may be sent to a level three inspection station. At the level three station, the bag may be inspected with a slower inspection device, than a level one system, that may use a different detection methodology to the level one system.
One example of a level three inspection device may be a CT scanner. CT scanners are usually successful in identifying explosives inside a bag when the explosives are present in large amount. The CT scanner may measure the mass density of the examined object. The CT scanner may be set up to communicate with the level one system in order to interrogate a specific object or region of interest, that was identified in the bag by the level one system. However, CT scanners can be expensive and slow.
Another example of a device that may be used as a level three detection device may be a multi-probe tomography system such as that described in U.S. Pat. No. 5,642,393, herein incorporated by reference.
On average, a level three device may tend to clear less than half of the objects it inspects. Thus, approximately 0.05%–0.25% of the baggage may need to be sent to a level four area. A level four area may be defined as reconciliation of the bag with the owner, which may often be difficult. If reconciliation is not possible, the bag may be confiscated and additional problems may arise, such as, termination of the flight that the bag was to be on.
While the above system can perform adequately, there is still a need for a device that may be used, for example, as a level three device that can reliably detect various explosives and other contraband having different shapes and locations in the item under inspection.